Field
The disclosed technology relates to a manufacturing titanate nanostructure such as nanosheets, nanotube, nanofiber, nanocomposite (0D nanoparticle and 1D nanotube) structures, via in situ and ex situ processes based on titanium precursors.
Background
Titanium nanomaterials comprising a metal or metal oxide with one dimensional nanostructures, or nanomaterials comprising a composite material of an inorganic or organic compound and a metal oxide having nanotube structures have been found to have different physical and chemical properties from the corresponding bulk materials.
Methods for producing titanate nanotubes and other nanostructured materials under hydrothermal conditions have been reported. Published European Application No. 0 832 847 describes this conventional method for producing titanate nanotubes with a diameter 5˜50 nm by alkaline treatment of titanium oxide. In this method, titanium dioxide is heated with sodium hydroxide for 1-50 hours at a temperature of 18 to 160° C. The product obtained is washed with water and neutralized. To increase the crystallinity of the product, thermal treatment is done in the range of 300-800° C. for 60 to 160 minutes. At a temperature above 180° C., no nanotubes with required characteristics are obtained.
After this, further studies have been carried out to apply this method to other materials. For example, US Published Patent Application No. 2010/0284902 described this method for producing alkaline sodium titanate nanotubes to obtain or control morphology of nanostructural titanates. Bavykin, D. V., et al, Adv. Mater, 2006, 18, 2807, Sun X., Li Y., Chem, Eur. J. 2003, 9, 2229 and Ma R., Sasaki T., et al., Chem. Commun 2005, 948, have demonstrated that sodium titanates nanotubes show high ion exchange reactivity towards alkali metal cations for renewable energy applications. In contrast, US Published Patent Application No. 2009/0117028 describes hydrothermal treatment methods which have longer reaction times. Hydrothermal treatment time duration varies from 10 hours to as high as 72 hours, with 24 and 48 hours being typical; however, in some cases, these extended reaction times could be impractical. Therefore, a faster synthesis is desired. In this case, microwave irradiation is considered to be the most efficient and distinct heating method, because of very short reaction time and low energy consumption needed for the reactions, compared to this conventional hydrothermal method.
V. Rodriguez-Gonzáleza, et al., J. Mol. Cat. A: Chem., 2012, 353-354, 163-170, described microwave hydrothermal treatment method for producing silver assisted titanate nanotubes. First they prepared Ag/TiO2, which was mixed with 10M NaOH, followed by microwave irradiation at 150° C., 195 watts for 4 hours. After washing with 5M HCl, the resulting products were washed with water to keep pH˜7 followed by drying at 95° C. for 12 hours. Their initial sample preparation method, before microwave irradiation, takes a longer time and their study did not consider microwave irradiation power, pressure, time and temperature which are the important and critical parameter in synthesizing nanotubes and nanostructured materials.
The transformation mechanism of titanate nanotubes and other nanostructured materials demonstrate insights to the structure and morphology of these materials and provide guidance to facilitate the design of nanomaterials useful for specific applications, described in US Published Patent Application No. 2009/0117028. Titanate nanotubes are usually formed by rolling nanosheets proposed by Renzhi Ma, et al., J. Phys. Chem. B 2004, 108, 2115-2119, and by B. D. Yao, et al., App. Phy. Lett., 2003, 82, 2. The references describe nanosheets formed at low temperature hydrothermal reactions and nanotube formed at higher temperature; however, formation mechanism of nanosheets from precursors are not clearly demonstrated. Jianjun Yang, et al., Dalton Trans., 2003, 3898-3901, described the combination of two theories; namely nanosheets exfoliation from the precursor or partial dissolution of precursor in concentrated sodium hydroxide solutions followed by the nucleation of sodium titanate followed by their subsequent growth. S. Zhang, et al., Phy. Rev. Lett., 2003, 91, 25, 256103-1, proposed that a mechanical tension arises during formation due to the width difference between two layers of nanosheets. In the case of structure and composition, there is some confusion. Tomoko Kasuga, et al., Langmuir, 1998, 14, 3160-3163, proposed final structure titania nanotube found by acid washing. Wenzhong Wang, et al., J. Mater. Res., 2004, 19, 2; Y. Q. Wang, et al., Chem. Phy. Lett., 2002, 365, 427-431; and G. H. Du, et al., App. Phy. Lett., 2001, 79, 22-26, proposed end product is hydrogen titanate (H2Ti3O7) nanotubes without the need for washing. Therefore, understanding the exact mechanism of nanotube formation from nanosheets as well as their chemical structure and compositions is not clearly defined.
General interest for transition metal doping of titanate nanotubes and nanostructured materials is enormous because the surface chemistry changed by transition metal doped titanate nanostructures is a key factor to tube up the properties of catalyst and catalytic performance. Structural details related to catalytic properties and active adsorption sites in metal doped titanate nanotubes or other nanostructured materials are not yet known and required for further research. Ajayan et. al., Nature, 1995, 375, 564-567, have reported that metal oxide nanotube materials based on carbon nanotubes can be used as a template. This method contains carbon or another impurity which can be major obstacle in its application and also synthesis cost is high due to consuming template. U.S. Pat. No. 7,592,039 describes mass production of metal oxide nanotube materials but those are all of metal oxide thin film using a conventional template method.